Revert r161710 and r161714

Revert following changes:
- r161714: "Remove wrong overload."
- r10687371674148261103: "Make ExecutionContext not own refcount."

It turns out ExecutionContext needs its own ref count after all
because Worker refers the context and the primary owner of the context
cannot delete it by its own right.

TBR=abarth
BUG=305497

Review URL: https://codereview.chromium.org/62493011

git-svn-id: svn://svn.chromium.org/blink/trunk@161782 bbb929c8-8fbe-4397-9dbb-9b2b20218538